1. Technical Field of the Invention
The present invention relates to a metal-to-metal seal assembly and, more particularly, to such an assembly for use in oil and gas production apparatus located in a well.
2. Description of Related Art
It is critically important to properly seal certain components in oil and gas wells during the operation of downhole tools, after completion and testing of the well, and during production. For example, expansion joints, often referred to as xe2x80x9cpolished bore receptacles,xe2x80x9d can be connected in the production tubing string in a completed well to compensate for changes in the axial length of the tubing string due to the effects of relatively large temperature changes in the well. Failure to compensate would otherwise cause a compression deformation or tensile failure. A typical polished bore receptacle includes two tubular members disposed in a telescoping relationship that is move relatively to each other in an axial direction in response to temperature variations, and a continuous dynamic seal is provided between the two members to prevent fluid leakage between the sliding surfaces of the two members.
Elastomer seals have been used in a variety of sealing applications in oil and gas wells, including use in the polished bore receptacles described above. However, the elastomer may lose its resiliency or shape memory after some use, which is necessary for the seal to oppose the imposed forces thereon. Also, elastomer seals tend to deteriorate with exposure to the downhole chemical and relative high temperature environments for long periods of time. Further, significant abrasion of the seal material will occur by the forces generated when there is relative movement between the two members being sealed, as is the case with polished bore receptacles. Although these deficiencies can be compensated for to a certain degree by preloading the seal, the preloading force becomes less as more and more of the seal material abrades, ultimately causing seal leakage and failure.
Therefore, to overcome these problems, metal-to-metal seals have evolved since they, for the most part, do not lose their resiliency and shape memory and are not affected by hostile environments. However, metal-to-metal seals are normally only used as static seals or as safety backup seals since the seal must remain stationary and must be under constant compression to insure that it is not compromised. Therefore, these metal-to-metal seals are not suitable for use in dynamic sealing applications, including the polished bore receptacles described above.
Prior art patents have addressed the need for metal-to-metal seals to some extent. For example, U.S. Pat. No. 5,662,341 which issued to the present inventors, discloses an earlier type of metal to metal seal assembly. FIGS. 1 and 2 illustrate this prior art seal. Referring to FIG. 1 of the drawings, the reference numeral 10 refers in general, to the expansion joint, or polished bore receptacle, of the present invention which is adapted to be connected between two tubular sections (not shown) forming a portion of production tubing string in an oil or gas well. The assembly 10 consists of an inner mandrel 12 telescopically received in an outer tubular member 14. It is understood that the inner bore of the outer tubular member 14 is polished and that the entire lengths of the overlapping end portions of the mandrel 12 and the tubular member 14 are not shown in their entirety for the convenience of presentation.
The respective distal end portions of the mandrel 12 and the tubular member 14 are threaded for connection to the two tubular sections of the tubing string (not shown) in coaxial alignment. The respective inner bores of the mandrel 12, the tubular member 14 and the tubing string sections are aligned in a coaxial relationship and thus provide a continuous passage for the flow of production fluid upwardly, as viewed in FIG. 1, through the lower portion of the tubing string, the tubular member 14, the mandrel 12 and the upper portion of the string.
The mandrel 12 has a stepped outer surface and the tubular member 14 has a stepped inner surface. As a result, a shoulder 12a is defined on the outer surface of the mandrel 12 which, in the assembled condition of the assembly 10 as viewed in FIG. 1, abuts against a corresponding shoulder formed on the tubular member 14. An annular cross-sectional space is defined between the outer surface of the mandrel 12 and the inner surface of the tubular member 14, which space extends below the shoulder 12a and the corresponding shoulder of the tubular member 14. The reference numeral 14a refers to a shoulder defined on the inner surface of the tubular member 14 at which the inner diameter of the latter member increases in a direction from the upper portion to the lower portion, for reasons to be described.
A locking mandrel 16 extends over the upper end portion of the tubular member 14 and has a inwardly-directed flange 16a which engages the end of the latter member. A plurality of angularly-spaced, radially-extending openings 16b (only one of which is shown in the drawing) are formed through the locating mandrel 16 and align with corresponding openings in the tubular member 14 and the mandrel 12. A plurality of pins 18 are provided which, during assembly, pass through the openings 16a respectively, and extend in the respective aligned openings in the tubular member 14 and the mandrel 12. This locates the mandrel 12 relative to the tubular member 14 in the position shown and prevents relative axial movement therebetween. The pins 18 are adapted to shear in response to a predetermined shear force between the mandrel 12 and the tubular member 14, in a conventional manner. A threaded pin 20 extends through a threaded opening in the locating mandrel 16 and into a notch 14b formed in the upper surface of the tubular member 14 to secure the locating mandrel to the member.
A bearing ring 24 extends around the mandrel 12 and in the annular space between the mandrel 12 and the tubular member 14. A wire ring 26 is used to secure the bearing ring 24 in the position shown. Another bearing ring 28 also extends around the mandrel 12, in the latter annular space, and above the bearing ring 24 in a slightly spaced relation thereto.
A seal ring 30 extends around the mandrel 12, in the annular space between the mandrel 12 and the tubular member 14, and between the bearing rings 24 and 28. As better shown in FIG. 2, the seal ring 30 has a substantially xe2x80x9cCxe2x80x9d-shaped cross section and, in the assembled portion shown in FIG. 1, the open portion of the C faces downwardly, i.e., in a direction facing the production fluid as it flows upwardly through the assembly 20. The xe2x80x9cCxe2x80x9d configuration defines two parallel sections 30a and 30b which abut the tubular member 14 and the mandrel 12, respectively, as will be described in further detail. The seal ring 30 is preferably fabricated from a metal material, and the height of the ring is slightly more than the height of the annular space between the mandrel 12 and the tubular member 14 in the portion of the annular gap in which the ring 30 is installed.
A coiled spring 32 is disposed within the seal ring 30 and extends for its entire circumference. The purpose of the spring 32 is to preload the seal ring 30 as will be described.
A coiled spring 32 is disposed within the seal ring 30 and extends for its entire circumference. The purpose of the spring 32 is to preload the seal ring 30 as will be described. The seal ring 30 has a major diameter defining its circumference, as well as a cross sectional diameter which defines the cross-sectional area of the seal ring 30.
The assembly 10 is assembled by initially placing the bearing ring 24 over the outer surface of the mandrel 12 and aligning the groove in the inner surface of the ring 24 with the complementary groove in the outer surface of the mandrel 12. The wire 26 is then threaded through a tap or opening (not shown), in the ring 24 and into the aligned grooves to secure the ring 24 against axial movement relative to the mandrel 12. The seal ring 30 and the bearing ring 28 are then advanced over the mandrel 12 until the seal ring extends between the bearing rings 24 and 28 in close proximity thereto. The mandrel 12 is then inserted, or stabbed, into the upper end of the tubular member 14, with the mule shoe guide 34 aiding in properly aligning the mandrel and the tubular member. The mandrel 12 is then advanced relatively to the tubular member 14 in a downward direction as viewed in FIG. 1 until the shoulder 12a of the mandrel 12 abuts the corresponding shoulder of the tubular member 14. During this movement, the bearing rings 24 and 28 and the seal ring 30 pass the shoulder 14a of the tubular member 14 and thus encounter the above-mentioned portion of the inner surface of the tubular member 14 in which the inner diameter of the latter member increases and the height of the annular space between the mandrel 12 and the tubular member 14 is slightly less that the height of the cross section of the ring 30. Thus, when the mandrel reaches its assembled position shown in FIG. 1, the ring 30 is secured between the mandrel and the tubular member in a strong interference fit. The locking mandrel 16 is then placed over, and secured to, the end portion of the tubular member 14, and the shear pins 18 are inserted into their respective aligned openings to secure the assembly 10 in its assembled position, ready for installation in the well.
In operation, the assembly 10 is assembled in the manner discussed above and is connected between two sections of production tubing and lowered into the well to be serviced. The production fluid passes upwardly through the continuous bore established by the respective bores of the lower tubular string, the tubular member 14, the mandrel 12 and the upper tubular string. The assembly 10 expands and contracts in an axial direction as a result of corresponding relative telescoping movement between the mandrel 12 and the tubular member 14 in response to corresponding changes in the temperature in the well.
Although the fluid will attempt to enter the annular space between the mandrel 12 and the tubular member 14, the seal ring 30, with assistance from the bearing rings 24 and 28, prevents any ingress. More particularly, and with reference to FIGS. 1 and 2, the fluid will enter the opening defined in the xe2x80x9cCxe2x80x9d cross section of the ring, i.e., between the respective ends of the sections 30a and 30b. The fluid pressure acting against the seal ring 30 will be constant in the center of the xe2x80x9cCxe2x80x9d shaped cross-sectional portion of the ring, as well as between the tubular member 14 and the section 30a of the ring, and between the mandrel 12 and the section 30b of the ring. Also, the ring 30 is secured between the mandrel 12 and the tubular member 14 in a strong interference fit and the spring 32 functions to maintain the shape of the ring 30 during loading. As a result of all of this, the sections 30a and 30b of the ring 30 are prevented from getting bent radially inwardly away from the tubular member 14 and the mandrel 12, respectively, thus preventing collapsing of the ring. Thus, the pressure across the annular gap between the mandrel 12 and the tubular member 14 is equal and a strong seal is established. Also, the seal ring 30 is adapted for slight movement up and down as needed to accommodate the relative axial movement of the mandrel 12 and the tubular member 14.
There are times when a moving piston must be appropriately sealed. Thus, a need exists for a metal-to-metal piston system that allows for the application of a piston conveyed force to be applied to a linearly moving body while maintaining a metal to metal seal between-the piston and the outer and inner cylinders. The metal to metal seal will keep a constant load against the piston and the cylinder throughout the full stroking operation of the piston.
This metal-to-metal piston system allows for the application of a piston conveyed force to be applied to a linearly moving body while maintaining a metal to metal seal between the piston and the outer and inner cylinders. The metal to metal seal will keep a constant load against the piston and the cylinder throughout the full stroking operation of the piston.
This metal to metal piston system consists of a metal ring with metal to metal seals on both the outer diameter and the inner diameter. This piston will be installed inside a cylinder and it will have a shaft run inside of it. When assembled in this manner, the metal seals on the piston inner diameter will form an interference seal between the piston""s outer diameter and the cylinder""s inner diameter. Application of pressure on the lower end of the piston will cause the piston to travel linearly inside the cylinder. This system also contains elastomeric O-rings above the piston. These O-rings are used as wipers to maintain a clean surface for the metal to metal seals to seal on during the travelling process. The seal will also be maintained once the piston has traveled to its full extent.
The conventional method for sealing the piston between the cylinder and the shaft is with the use of elastomeric O-rings. These O-rings can degenerate through exposure to well fluids. Over extended periods of time, these O-rings seals can also loose sealing integrity. Elastomeric seals are also adversely effected by temperature fluctuations. The metal seals that are used in the metal to metal sealing piston system, on the other hand, are much more resilient to well fluids and temperature fluctuations.